Generally speaking, photovoltaic systems are implemented to convert light energy into electricity for a variety of applications. Power production by photovoltaic systems may offer a number of advantages over conventional systems. These advantages may include, but are not limited to, low operating costs, high reliability, modularity, low construction costs, and environmental benefits. As can be appreciated, photovoltaic systems are commonly known as “solar cells,” so named for their ability to produce electricity from sunlight.
Conventional solar cells convert light into electricity by exploiting the photovoltaic effect that exists at semiconductor junctions. Accordingly, conventional solar cells generally implement semiconductor layers to produce electron current. The semiconductor layers generally absorb incoming light to produce excited electrons. In addition to the semiconductor layers, solar cells generally include a glass cover or other encapsulant, an anti-reflective layer, a front contact electrode to allow the electrons to enter a circuit, and a back contact electron to allow the electrons to complete the circuit when excited electrons are injected into the semiconductor layer due to light exposure.
One particular type of solar cell is a dye-sensitized solar cell. A dye-sensitized solar cell generally uses an organic dye to absorb incoming light to produce excited electrons. The dye sensitized solar cell generally includes two planar conducting electrodes arranged in a sandwich configuration. A dye-coated semiconductor film separates the two electrodes which may comprise glass coated with a transparent conducting oxide (TCO) film, for example. The semiconductor layer is porous and has a high surface area thereby allowing sufficient dye to be attached as a molecular monolayer on its surface to facilitate efficient light absorption. The remaining intervening space between the electrodes and the pores in the semiconductor film (which acts as a sponge) is filled with an organic electrolyte solution containing an oxidation/reduction couple, such as triiodide/iodide, for example.
One exemplary technique for fabricating a dye-sensitized solar cell is to coat a conductive glass plate with a semiconductor film such as titanium oxide (TiO2) or zinc oxide (ZnO), for example. The semiconductor film is saturated with a dye and a single layer of dye molecules self-assembles on each of the particles in the semiconductor film, thereby “sensitizing” the film. To complete the solar cell, a liquid electrolyte solution containing triiodide/iodide is introduced into the semiconductor film. The electrolyte fills the pores and openings left in the dye-sensitized semiconductor film. To complete the solar cell a second planar electrode with low overpotential for triiodide reduction is implemented to provide a structure having a dye-sensitized semiconductor and electrolyte composite sandwiched between two counter-electrodes.
Conventional dye sensitized solar cells may be fabricated using planar layered structures, as set forth above. The absorption of light by the dye excites electrons in the dye which are injected into the semiconductor film. The electrons travel through the semiconductor film by a “random walk” through the adjacent crystals of the film towards one electrode, while oxidized dye cations, or ions formed by separation of the electrons in the dye, are reduced by the transfer of electrons from a reduced ion (commonly iodide) in the electrolyte, leading to the production of an oxidized species in the electrolyte (commonly triiodide) that migrates toward the opposite electrode where a catalyst supplies the missing electron to complete the circuit. During the random walk of the electron to the electrode, the electron may travel a significant distance, and the electron may be lost by combining with a component of the electrolyte solution, also known as “recombination.” Under irradiation by sunlight the density of electrons in the semiconductor may be very high such that such losses significantly reduce the maximum voltage and therefore the efficiency achievable by the solar cells. It may be advantageous to reduce the likelihood of recombination by reducing the travel path of the electron through the semiconductor and thereby reduce the length of time it takes for the electron to diffuse through the semiconductor to the conductive oxide of the electrode. One technique for reducing the travel distance of the electron is to reduce the thickness of the semiconductor film and thus, the distance the electron has to travel to reach an electrode. Disadvantageously, reduction in the thickness of the semiconductor film may reduce the light absorption in the dye, thereby reducing the efficiency of the solar cell.
Similarly, the oxidant (commonly triiodide ions) created by separation of the electrons travel to the back electrode to complete the circuit. Because the ion may originate from any area of the electrolyte solution that is in contact with the dyed surface of the semiconductor material, the ion may have to travel a long distance (e.g. from the front electrode to the back electrode) through the labyrinth created by the porous structure of the semiconductor material. The electrolyte solution is typically made in an organic solvent. Generally speaking, less volatile solvents with a high boiling point, including ionic liquids, are more viscous and impede the diffusion of ions to the point where the diffusion limits the power output and hence the efficiency of the solar cell. Such solvents may be advantageous in providing cell longevity, especially for cells fabricated on a polymer substrate, because polymer substrates may allow less viscous solvents having a low boiling point to diffuse out of the solar cell over time. Decreasing the travel distance of the ions may advantageously reduce the limitations caused by the slow diffusion of the ions. However, as previously described, reducing the thickness of the semiconductor film may reduce the light absorption potential of the dye.
Thus, while it may be advantageous to increase the thickness of the semiconductor film and thereby the surface area of the film to provide increased light absorption, the thicker the semiconductor film, the greater the distance the electrons and ions may travel to reach an electrode. Although longer light paths may be desirable to facilitate greater light absorption, the losses due to the increased recombination of the electrons into the semiconductor layer, as well as limits to current caused by slow ion diffusion through the electrolyte in the semiconductor pores, make the increased thickness in the semiconductor film disadvantageous since it may produce a less efficient solar cell.